Laser illumination system

ABSTRACT

The illumination energy of an excimer laser is measured and adjusted to always effect illumination at constant energy. A laser beam output from an optics is reflected by a mirror, and applied to a sample. A beam profiler is disposed behind the mirror to measure the energy of an illumination laser beam. An energy attenuating device disposed between another mirror and the optics is operated based on the measurement value so that the energy of the laser beam applied to the sample is kept constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the configuration of a laser apparatusused for a semiconductor device manufacturing process and otherpurposes. In particular, the invention relates to a laser apparatus usedto improve or restore, by application of laser light, the crystallinityof a semiconductor material partially or fully made of an amorphouscomponent, a substantially intrinsic, polycrystalline semiconductormaterial, or a semiconductor material whose crystallinity has beenlowered being damaged by ion application, ion implantation, ion doping,or the like.

The invention also relates to a laser illumination system used for alow-temperature process for producing TFTs that are used in a liquidcrystal display device and, more specifically, to a technique forforming, on the same substrate, thin-film transistors having a largemobility disposed in a peripheral circuit area and a number of thin-filmtransistors having uniform characteristics disposed in association withrespective pixels.

2. Description of the Related Art

A panel used in a process for manufacturing a liquid crystal displaygenerally has a peripheral circuit area and a pixel area. The peripheralcircuit area has a role of controlling the value of a current flowingthrough each pixel. As semiconductor devices in the peripheral circuitarea have a larger mobility, the circuit configuration of the displaycan be made simpler and the display is allowed to operate at higherspeed. On the other hand, pixels have a role of holding information sentfrom drivers. If semiconductor devices in the pixel area do not have asufficiently small off-current, they cannot hold such information.Further, if off-current values are much different from one pixel toanother, the pixels display differently the same information sent fromthe drivers. For the above reasons, a technique is now required whichallows semiconductor devices having different characteristics to beselectively formed on the same substrate.

In recent years, extensive studies have been made to decrease thetemperature of semiconductor device manufacturing processes. This islargely due to the necessity of forming semiconductor devices on aninsulative substrate made of glass or the like, which substrate isinexpensive and has high workability. In general, when a glass substrateis exposed to a high-temperature atmosphere of 600° C. or more, itexpands or is deformed, for instance. Therefore, it is now desired thatthe temperature of semiconductor device manufacturing processes bereduced as much as possible. The decrease of the process temperature isalso required from miniaturization and multi-layering of devices.

In semiconductor device manufacturing processes, it is sometimesnecessary to crystallize an amorphous component of a semiconductormaterial or an amorphous semiconductor material, to return to theoriginal crystalline level the crystallinity of a semiconductor materialwhich has been lowered by ion application, or to improve thecrystallinity of an already crystalline semiconductor material. This isbecause if such materials are crystallized, the mobility of resultingsemiconductor devices can be made very large.

Conventionally, thermal annealing is used for the above purpose. Wheresilicon is used as a semiconductor material, the crystallization of anamorphous material, the restoration of an original crystallinity level,the improvement of crystallinity, etc. are attained by performingthermal annealing at 600 to 1,100° C. for at least several tens ofhours.

In general, the processing time of such thermal annealing can beshortened as the temperature increases. On the other hand, almost noimprovement is obtained at a temperature lower than 500° C. Therefore,to decrease the process temperature, it is necessary to replace aconventional thermal annealing step with some other proper step.

There is known, as one of the techniques for satisfying the above needare, a technique of performing various kinds. of annealing by laserlight illumination. Since laser light can supply high energy equivalentto that of thermal annealing to a desired, limited portion, thistechnique has an advantage that it is not necessary to expose the entiresubstrate to a high-temperature atmosphere. In general, there have been.proposed two laser light illumination methods.

In a first method, a CW laser such as an argon ion laser is used and aspot-like beam is applied to a semiconductor material. A semiconductormaterial is crystallized such that it is melted and then solidified soondue to a sloped energy profile of a beam and its movement.

In a second method, a pulsed oscillation laser such as an excimer laseris used. A semiconductor material is crystallized such that it isinstantaneously melted by application of a high-energy laser pulse andthen solidified.

The first method of using a CW laser has a problem of long processingtime, because the maximum energy of the CW laser is insufficient andtherefore the beam spot size is at most several millimeters.

In contrast, the second method using a pulse oscillation laser canprovide high mass-productivity, because the maximum energy of the laseris very high and therefore the beam spot size can be made as large asseveral square centimeters.

However, to process a single, large-area substrate with an ordinarysquare or rectangular beam, the beam needs to be moved in the fourorthogonal directions, which still remains to be solved from theviewpoint of mass-productivity.

This aspect can be greatly improved by deforming a beam into a linearshape that is longer than the width of a subject substrate, and scanningthe substrate with the beam.

The remaining problem is insufficient uniformity of laser lightillumination effects. Pulsed oscillation lasers as represented by anexcimer laser in which laser oscillation is obtained by gas dischargehave a tendency that the energy somewhat varies from one pulse toanother. Further, the degree of the energy variation also varies withthe output energy. In particular, when illumination is performed in anenergy range where stable laser oscillation cannot be obtained easily,it is difficult to perform laser processing with uniform energy over theentire substrate surface.

Another problem associated with the use of a pulsed oscillation laser isthat the laser light energy decreases as the laser is used over a longtime, which attributes to degradation of a gas necessary for laseroscillation. This does not appear to be a serious problem because thelaser light energy can be increased by raising its operation level.However, in practice, raising the operation level is not preferablebecause once the operation level is changed, it takes some time for thelaser light energy to be stabilized.

By the way, it is conventionally very difficult to produce, only withlaser light illumination, a crystalline silicon film having such a largemobility as enables fast operation of a liquid crystal display. In viewof this, a method of improving the crystallinity after laser lightillumination has been proposed in which thermal annealing forcrystallization is performed at about 550° C. for several hours beforethe laser light illumination. Although this method can attain a mobility(about 20 cm²/Vs) as required for the pixel area and the off-current issmall (about 10⁻¹² A) and has a small pixel-to-pixel variation (on thesame order), it cannot provide a mobility (more than 100 cm²/Vs) asrequired for the driver area.

We have already proposed the following method for solving this problem.

In the first step, a metal element such as Ni is added to asemiconductor material that is deposited on a glass substrate. Varioussubstances other than Ni can be used as long as they serve as nucleiwhen the semiconductor material is crystallized. However, according toour experiments, in the case where the semiconductor material isamorphous silicon, the addition of Ni effectively produced silicon filmshaving the best crystallinity. The following description will be limitedto the case where the impurity is Ni.

Among various methods of adding the impurity is a method of applying anickel acetate salt solution to the surface of a semiconductor material.

In the second step, the Ni-added semiconductor material is kept at ahigh temperature. Where the semiconductor material is an amorphoussilicon thin film, a crystalline silicon film is produced by keeping theNi-added amorphous silicon thin film for 4 hours in an atmosphere of550° C. During this heat treatment step, Ni penetrates through thesemiconductor material and crystal growth proceeds with Ni serving asnuclei. Thus, a crystalline film of the semiconductor material isproduced.

In the third step, a film having better crystallinity is produced byapplying laser light to the semiconductor material. The above-describedlinear laser light is used in this step. The laser light illumination isperformed such that before application of strong pulsed laser light,preliminary illumination is conducted with weaker pulsed laser light.This allows formation of a semiconductor film having highly uniformcrystallinity. The two-step illumination is effective in suppressingdegradation of the uniformity of the film surface due to laser lightillumination.

The reason why the preliminary illumination is effective in obtaining auniform film is that a crystalline silicon film obtained by thepreceding steps still includes many amorphous portions in which theabsorption factor of laser light energy is much different from that of apolycrystalline film. That is, the residual amorphous portions arecrystallized by the first illumination, and the total crystallization isaccelerated by the second illumination. The two-step illumination isvery advantageous, and can greatly improve the characteristics ofcompleted semiconductor devices.

To reduce the degree of abrupt temperature change of a silicon filmsurface due to laser light illumination, it is preferred that during thelaser light illumination a substrate be kept at 100 to 600° C. It isknown that in general an abrupt change in environmental conditionsimpairs the uniformity of a substance. By keeping the substratetemperature high, the degradation of the uniformity of a substratesurface due to laser light illumination can be minimized. No particularatmosphere control is performed; i.e., the illumination is performed inthe air.

The mobility of a crystalline film thus produced depends on thesemiconductor material and the laser light energy. Where thesemiconductor material is silicon, a crystalline silicon film having amobility of more than 100 cm²/Vs can be obtained. Although the mobilitygenerally increases as the laser light energy is increased, it starts todecrease at a certain high energy level.

Although thin-film transistors formed by using a crystalline siliconfilm produced by the above method have a high mobility, they have largeoff-current values that vary very much from one transistor to another(two to five orders). (The off-current variation becomes almostunnoticeable if the laser light energy is so reduced as to provide amobility of 20 cm²/Vs.) The variation of off-current values adverselyaffect the pixels, and causes point defects and line defects in acompleted liquid crystal display.

As described above, a pixel-to-pixel variation of the off-current in thepixel area causes fatal defects for the operation of a liquid crystaldisplay. However, it has been proved that a variation of off-currentvalues of thin-film transistors disposed in the peripheral circuit areadoes not have large influences on the operation of a liquid crystaldisplay. It has also been proved that while a large mobility (largerthan 100 cm²/Vs) is required for the peripheral circuit area, arelatively small mobility (about 20 cm²/Vs) is sufficient for the pixelarea.

Based on the above discussions, an appropriate laser light applicationscheme is such that high-energy laser light is applied to the peripheralcircuit area and low-energy laser light is applied to the pixel area(see FIG. 8).

However, the scheme of individually applying laser beams of differentenergies to the peripheral circuit area and the pixel area makes thelaser light illumination step complex and time-consuming. For example,if the peripheral circuit area is illuminated while the pixel area ismasked, and vice versa, this process takes long time and becomes complexbecause of two times of illumination. Further, if the above-describedtwo-step illumination is employed, this scheme needs four times ofillumination.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, by solving the aboveproblems, a laser illumination system which can perform illumination atconstant laser energy even using a pulsed oscillation laser.

Another object of the invention is to provide an apparatus which allowsthe above-described laser light illumination method to be performed inshort time in a simple way.

To attain the above objects, the invention is characterized in that anenergy attenuating device as represented by a light attenuation filterand an energy measuring device as represented by a beam profiler areused in combination.

That is, in the invention, a laser is oscillated at an output level atwhich the laser operates in as stable a state as possible. Further, byadditionally using the energy attenuating device, the laser lightintensity is adjusted to illuminate an object at an optimum energy.

In the invention, it is preferred that the energy attenuation factor ofthe energy attenuating device be continuously variable. But it may bediscretely variable. That is, the invention is summarized such that thelaser light energy as output is set higher than the above-mentionedoptimum energy and the laser light energy is adjusted to the optimumenergy by using the energy attenuating device. In doing so, the laser iscaused to operate in an energy range where it can oscillate in as stablea state as possible. As the laser continues to oscillate over a longtime, the laser light energy tends to decrease. In the invention, thisenergy reduction is compensated by adjusting the energy attenuatingdevice. That is, the invention is characterized by enabling laser lightillumination to be always performed at a constant energy by attenuating,in the initial stage, the laser light energy by the energy attenuatingdevice by an amount equal to the energy reduction in the final stage,and gradually reducing the attenuation factor as the laser lightillumination proceeds. This is why it is preferred that the energyattenuating device is continuously variable.

According to another aspect of the invention, a laser illuminationapparatus producing a large-area beam spot is combined with a device(hereinafter called an energy attenuating device) capable of attenuatingthe energy of the large-area laser beam at different attenuation factorsfor respective portions of the beam. By virtue of using such an energyattenuating device, laser light illumination is conducted only once,which contributes to increase of the throughput. Even if the inventionis combined with the above-described two-step illumination, illuminationis conducted only two times.

If the energy measuring device is added to the above system, the laserlight energy can be controlled more precisely. In general, pulsed laserssuch as an excimer laser has a tendency that a certain degree ofvariation occurs in the laser light energy even if the laser output iskept constant. This problem may be solved by, for instance, changing thelaser output itself (operation level) in accordance with a variation ofthe laser light energy, or by changing the energy attenuation factor ofthe energy attenuating device. The former method is not appropriate inthe case where the laser light energy needs to be controlled veryprecisely, because the laser oscillation itself becomes unstable whenthe laser output is changed. On the other hand, since the latter methoddoes not change the laser output, the laser oscillation does not becomeunstable. Thus, the latter method is advantageous over the formermethod. However, the latter method needs to use the energy attenuatingdevice having a variable energy attenuation factor.

The laser illumination system having the above configuration enablesformation of a silicon film having regions of different electricalcharacteristics on the same substrate by changing the laser lightillumination energy. Further, by using the above-described energyattenuating device, it becomes possible to perform laser processing withthroughput equivalent to that as would be obtained when laserillumination is conducted without changing the laser light illuminationenergy. By forming a number of TFTs on the thus-produced silicon film,TFTs having a large mobility and TFTs having a small off-current can beformed on the same substrate. By using this technique, an active matrixliquid crystal display device can be constructed in which the peripheralcircuit area is constituted of TFTs having a large mobility and thepixel area is constituted of TFTs having a small off-current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general configuration of a laser annealing apparatus usedin a first embodiment of the invention;

FIG. 2 shows an optics used in the laser annealing apparatus of FIG. 1;

FIG. 3 shows a light attenuation filter;

FIG. 4 illustrates how to measure the energy of a linear laser beam;

FIG. 5 schematically shows the configuration of a laser illuminationsystem according to a second embodiment of the invention;

FIG. 6 shows how a laser beam is applied to a sample;

FIG. 7 shows a system for illumination with a laser beam; and

FIG. 8 schematically shows an active matrix liquid crystal panel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

First of all, a description will be made of an apparatus. FIG. 1 showsthe concept of a laser annealing apparatus used in this embodiment.Reference numeral 1 denotes a main body of the laser annealingapparatus. An oscillator 2 emits KrF excimer laser light (wavelength:248 nm; pulse width: 25 ns). Apparently other excimer lasers and othertypes of laser may also be used.

Laser light emitted from the oscillator 2 is reflected byfull-reflection mirrors 5 and 6, amplified by an amplifier 3, reflectedby full-reflection mirrors 7 and 8, and introduced into an optics 4.Although not shown in FIG. 4, an energy attenuating device is insertedbetween the mirror 8 and the optics 4. FIG. 3 shows the configuration ofthe energy attenuating device.

As shown in FIG. 3, the energy attenuating device is of a type in whicha single plate of filter faces a traveling laser beam and the energytransmission factor is changed by changing the angle formed by the beamand the filter.

The laser beam immediately before entering the optics 4 has arectangular shape of about 3×2 cm². It is shaped by the optics 4 into along and narrow (i.e., linear) beam having a length of 8 to 30 cm and awidth of 0 to 0.5 mm. The laser light as output from the optics 4 has amaximum energy of 1,000 mJ/shot.

The reason for shaping the laser light into the above long and narrowbeam is to improve its processibility. After being output from theoptics 4, the linear beam is reflected by the full-reflection mirror 9,and applied to a sample 11. Since the beam is longer than the width ofthe sample 11, the entire sample 11 can be illuminated with the laserbeam by moving the sample 11 in one direction. Therefore, the samplestage/driving device 10 can be made simple in configuration and can bemaintained easily. Further, the alignment operation in setting thesample 11 can be facilitated.

The sample stage 10 to which the laser beam is applied is controlled bya computer so as to move approximately perpendicularly to the linearlaser beam.

FIG. 2 shows an optical path in the optics 4. A laser beam entering theoptics 4 passes through a cylindrical concave lens A, a cylindricalconvex lens B, and horizontal and vertical fly-eye lenses C and D, sothat an original Gaussian profile is converted into a rectangularprofile. Thereafter, the laser beam is passed through cylindrical convexlenses E and f, focused by a cylindrical lens H through a mirror G(corresponds to the mirror 9 in FIG. 1), and applied to the sample 11.

The mirror G (corresponds to the mirror 9 in FIG. 1) is so constructedas to transmit a small part of the laser light energy. Therefore, bydisposing a beam profiler behind the mirror G, the laser light energycan be measured even while the laser beam is applied to the sample 11,i.e., on a real-time basis. Since the linear laser beam has a largearea, the energy is measured by moving the beam profiler within thelinear laser beam to scan it (see FIG. 4). In this manner, even theenergy profile of the linear laser beam can be measured.

The above apparatus is designed such that if the energy of the linearlaser beam has deviated from a preset energy by a predeterminedpercentage during laser light illumination, a signal is automaticallysent from a beam splitter to the energy attenuating device and the laserenergy is returned to the preset energy.

Embodiment 2

Although the mirror G being partially transmissive provides theadvantage of real-time measurement of the laser light illuminationenergy (first embodiment; FIG. 1), it has a disadvantage that part ofthe laser energy is lost. This embodiment is directed to an apparatusfor solving that problem. However, the apparatus of this embodimentcannot measure the energy of the linear laser beam on a real-time basis.

FIG. 5 shows a laser light illuminating portion of the apparatus of thisembodiment. A mirror P in FIG. 5 corresponds to the mirror G in FIG. 1.The mirror P is a full-reflection mirror, and a 4%-reflection mirror Qis disposed under the mirror P. An energy component reflected by themirror Q enters a beam profiler R. The mirror Q is smaller than themirror P, because the beam profiler measures a small area each time. Themirror Q moves in link with the beam profiler R, and is adapted to slideover a range longer than the linear laser beam. During laser lightillumination, the mirror Q and the beam profiler are slid outside thelinear laser beam. If an object of illumination is shorter than thelinear laser beam, the mirror Q may be placed at an end portion of thelinear laser beam which portion does not affect the illumination, inwhich case the energy can be measured even during laser lightillumination.

Embodiment 3

We have studied the method for improving the crystallinity of anamorphous or crystalline silicon or silicon compound film byilluminating it with laser light and, in particular, the manufacturingmethod of a film suitable as a material for a liquid crystal displaydevice, and propose the following apparatus that facilitates manufactureof such a film.

As in the first embodiment, this embodiment uses the laser annealingapparatus shown in FIG. 1. An energy attenuating device is inserted intothe output-side optical path of the optics 4 of FIG. 1. FIG. 6 shows theconfiguration of the energy attenuating device. A brief description willbe made of the mechanism of this device. Referring to FIG. 6, the lightattenuation factor of each of special light attenuation filters 104 and.105 is changed by changing the angle formed by the laser beam and thefilter. The transmission factor of each of the light attenuation filters104 and 105 is maximum when the filter is perpendicular to the laserbeam, and is reduced as the angle decreases from 90°. Although in FIG. 6the energy attenuating device consists of the two light attenuationfilters 104 and 105, there is no limitation on the number of lightattenuation filters. Several light attenuation filters may be used incombination depending on the laser light illumination object.

FIG. 6 schematically shows how the laser beam is applied, in which amirror 106 corresponds to the mirror 9 in FIG. 1, and a sample 101corresponds to the sample 11 in FIG. 1. A stage 103 corresponds to thestage 10 in FIG. 1, and can move in a direction indicated by arrow 113.

A description will be made of an example of forming a crystallinesilicon film on a glass substrate by laser light illumination by usingthe invention.

First, a glass substrate (for instance, Corning 7959) of 10 cm×10 cm isprepared. -A-2,000-Å-thick silicon oxide film (not shown) is formed onthe glass substrate by plasma CVD in which TEOS is used as a material.This silicon oxide film serves as a undercoat film for preventingimpurities from diffusing into a semiconductor film from the glasssubstrate side.

An amorphous silicon film is then deposited by plasma CVD. Low-pressurethermal CVD may be used instead of plasma CVD. The thickness of theamorphous silicon film is set at 500 Å. Apparently this thickness may beset as desired.

Subsequently, a silicon oxide film is formed on the surface of theamorphous silicon film by immersing the substrate in excessive waterammonia of 70° C. for 5 minutes. A liquid-phase nickel acetate salt isthen applied to the surface of the amorphous silicon film by spincoating. Nickel serves as an element for accelerating crystallizationwhen the amorphous silicon film is crystallized.

Although in this embodiment Ni is used as a metal element foraccelerating crystallization of silicon, there may also be used one or aplurality of elements selected from Fe, Co,.Ni, Ru, Rh, Pd, Os, Ir andPt.

Thereafter, hydrogen is removed from the amorphous silicon film bykeeping the substrate at 450° C. for one hour in a nitrogen atmosphere.This is to intentionally form dangling bonds in the amorphous siliconfilm, to reduce the threshold energy of crystallization later performed.The amorphous silicon film is then crystallized by performing a heattreatment of 550° C. and 4 hours in a nitrogen atmosphere. It is notedthat the action of Ni has enabled the crystallization to be performed ata temperature as low as 550° C.

A crystalline silicon film can be formed on the glass substrate in theabove manner. FIG. 6 shows a state that a glass substrate 101 formedwith a crystalline silicon film 102 is placed on a holder 103. In thisstate, a KrF excimer laser beam (wavelength: 248 nm; pulse width 25 ns)is applied to the crystalline silicon film 102 by using the apparatus ofFIG. 1. The crystallinity of the silicon film can be improved by thislaser beam illumination.

The laser beam is converted by beam shape conversion lenses into arectangular shape to produce an illumination beam area of 125 mm×1 mm.The sample is mounted on the stage 103, and its entire surface isilluminated by moving the stage 103 at a rate of 2 mm/s.

As for the laser beam illumination conditions, two-step illuminationconsisting of preliminary illumination of 100 to 300 mJ/cm² and mainillumination of 200 to 500 mJ/cm² is employed, and the pulse rate is setat 30 pulses/s. The two-step illumination is employed to minimizedegradation of the uniformity of a film surface due to laser beamillumination, to thereby form a film having better crystallinity. Thereason why the crystallinity is improved by this method has already beendescribed in the background part of this specification. The laser beamenergy is changed (for instance, from the preliminary illumination tothe main illumination) by changing the energy attenuation factor of theenergy attenuating device. This is conducted more easily than changingthe output energy of the laser itself.

The substrate temperature is kept at 200° C. during the laser beamillumination. This is to reduce the rate of increase and decrease of thetemperature of the substrate surface due to the laser beam illumination.While in this embodiment the substrate temperature is set at 200° C., inpractice it is set at an optimum value for laser annealing in a range of100° C. to 600° C. No particular atmosphere control is performed; thatis, the illumination is conducted in the air.

A specific laser beam illumination method will be described below, whichcan be used for either the first or second illumination of the twostep-illumination scheme. The amorphous silicon film 102 as the laserbeam illumination object of this embodiment is one that has been formedfor the purpose of producing a liquid crystal display, and is dividedinto a peripheral circuit area 111 and a pixel matrix area 112 by abroken line indicated in FIG. 6. The laser beam illumination isconducted while changing the laser beam energy with the broken line as aboundary. The energy attenuating device consisting of the lightattenuation filters 104 and 105 is used to change the laser beam energy.

As is understood from FIG. 6, the energy attenuating device can providedifferent energy attenuation factors for portions of the linear laserbeam on both sides of an arbitrary position. Further, even duringillumination with the linear laser beam, i.e., during laser beamscanning, the angles formed by the laser beam and the respective lightattenuation filters 104 and 105 can be changed independently of eachother, to enable illumination at different laser beam energies in asingle laser beam scanning operation. By properly combining the abovetwo features, laser processing of the above-mentioned energydistribution can be performed on the illumination object shown in FIG. 6by a single scan.

A specific illumination method is as shown in FIGS. 6 and 7. A linearlaser beam 107 comes from the left side of FIG. 6, passes through thelight attenuation filters 104 and 105, reflected by the path-foldingmirror 106, and applied to the illumination surface of the amorphoussilicon film 102.

The illumination object shown in FIG. 6 is divided into the peripheralcircuit area 111 and the pixel matrix area 112. FIG. 8 schematicallyshows the configuration of a liquid crystal panel having peripheralcircuit areas and a pixel area.

The stage 103 on which the glass substrate 101 formed with thecrystalline silicon film 102 is mounted is moved in a directionindicated by arrow 113. Incorporating a heater in its lower portion, thestage 103 can kept the substrate 101 at a desired temperature. Adescription will now be made of an example in which laser beams havingenergy densities of 300 mJ/cm² and 400 m/cm² are respectively applied tothe peripheral circuit forming area 111 and the pixel matrix formingarea 112. This is done according to the following procedure.

First, the laser is oscillated with its output set at a value largerthan 400 mJ/cm². Then, the angles of the light attenuation filters 104and 105 are adjusted so that they have different transmission factors(in this case, so that laser beams 108 and 109 have energy densities of300 mJ/cm² and 400 mJ/cm², respectively). Upon completion of thissetting, the laser beams 108 and 109 start to be applied to thesubstrate 101 from its left side. The stage 103 moves to the left asindicated by arrow 113. At a time instant when the linear laser beam hasjust finished illumination of the pixel matrix area 112, the angle ofthe light attenuation filter 105 is changed to the same angle as thelight attenuation filter 104. The illumination is continued thereafter.To perform the twostep illumination as described above, the aboveprocess may be conducted two times.

Embodiment 4

A description will be made of a case of adding the energy measuringdevice to the method of the first embodiment. The energy measuringdevice may be located anywhere as long as the energy of a laser beampassed through the energy attenuating device can be measured. Thisembodiment is directed to a case where the energy measuring device islocated at a position shown in FIG. 7. When a difference between anenergy measured by the energy measuring device and a preset energyexceeds a predetermined value, a signal is sent from the energymeasuring device to the energy attenuating device, which, in response,changes the energy attenuation factor. Thus, the laser beam energy canbe returned to the preset value. This system enables more precise energycontrol.

The energy measuring device may be added to the optics shown in FIG. 6such that the path-folding mirror 106 is made partially transmissive andthe energy measuring device is disposed on an optical path 110 on thetransmission side of the path-folding mirror 106. In this case, aseveral percentage of each of the laser beams 108 and 109 passes throughthe path-folding mirror 106, and travels along the optical path 110 toreach the energy measuring device, which measures energies of the laserbeams 108 and 109. Based on measured energy values, the angles of thelight attenuation filters 104 and 105 are controlled.

The laser beam illumination technique of the invention has made itpossible to perform laser processing while minimizing a variation of thelaser beam energy. As a result, it is expected that the reproducibilityof a laser processing step is improved, and that variations in thecharacteristics of products that are produced by a manufacturing processincluding a laser processing step are thereby reduced very much. Inparticular, the invention can effectively be applied to all the laserprocessing steps used in semiconductor device manufacturing processes,because such steps tolerate only a small laser beam energy range; thatis, a slight difference in energy causes great influences on thecharacteristics.

The laser illumination-system of the invention makes it possible togreatly change the characteristics of a part of a film to constitutesemiconductor devices. The invention can be applied to all the laserprocessing steps used in semiconductor device manufacturing processes.In particular, in the case of semiconductor devices are TFTs for aliquid crystal panel, it becomes possible to produce TFTs for driverdevices having a mobility larger than 100 cm²/Vs and TFTs for pixelshaving a small pixel-to-pixel variation in the characteristics,particularly off-currents of the pixels.

As a result, it becomes possible to increase the movement speed ofimages of a TFT liquid crystal display device produced by alow-temperature manufacturing process, and to decrease the rate ofoccurrence of defective substrates having point defects, line defects,or other defects.

As such, the invention is very useful from the industrial point of view.

What is claimed:
 1. A method for forming a semiconductor devicecomprising: forming a semiconductor film comprising silicon over asubstrate; introducing a CW laser light into an optical systemcomprising a mirror; shaping said CW laser light into a linear laserlight by said optical system; reflecting said linear laser light by saidmirror; transmitting said linear laser light through said mirror;introducing the transmitted linear laser light into a beam profiler; andirradiating the reflected linear laser light to said semiconductor film.2. A method for forming a semiconductor device comprising: forming asemiconductor film comprising silicon over a substrate; introducing a CWlaser light into an optical system comprising a mirror; shaping said CWlaser light into a linear laser light by said optical system; reflectingsaid linear laser light by said mirror; transmitting said linear laserlight through said mirror; introducing the reflected linear laser lightinto a beam profiler; and irradiating the transmitted linear laser lightto said semiconductor film.
 3. A method for forming a semiconductordevice comprising: forming a semiconductor film comprising silicon overa substrate; introducing a CW laser light into an optical systemcomprising a mirror; shaping said CW laser light into a linear laserlight by said optical system; reflecting said linear laser light by saidmirror; transmitting said linear laser light through said mirror;introducing the transmitted linear laser light into an energy measuringdevice; and irradiating the reflected linear laser light to saidsemiconductor film.
 4. A method for forming a semiconductor devicecomprising: forming a semiconductor film comprising silicon over asubstrate; introducing a CW laser light into an optical systemcomprising a mirror; shaping said CW laser light into a linear laserlight by said optical system; reflecting said linear laser light by saidmirror; transmitting said linear laser light through said mirror;introducing the reflected linear laser light into an energy measuringdevice; and irradiating the transmitted linear laser light to saidsemiconductor film.
 5. A method for forming a semiconductor devicecomprising: forming a semiconductor film comprising silicon over asubstrate; providing said semiconductor film with an element foraccelerating crystallization of silicon; crystallizing saidsemiconductor film provided with said element by a heat treatment;introducing a CW laser light into an optical system comprising a mirror;shaping said CW laser light into a linear laser light by said opticalsystem; reflecting said linear laser light by said mirror; transmittingsaid linear laser light through said mirror; introducing the transmittedlinear laser light into a beam profiler; and irradiating the reflectedlinear laser light to said semiconductor film.
 6. A method for forming asemiconductor device comprising: forming a semiconductor film comprisingsilicon over a substrate; providing said semiconductor film with anelement for accelerating crystallization of silicon; crystallizing saidsemiconductor film provided with said element by a heat treatment;introducing a CW laser light into an optical system comprising a mirror;shaping said CW laser light into a linear laser light by said opticalsystem; reflecting said linear laser light by said mirror; transmittingsaid linear laser light through said mirror; introducing the reflectedlinear laser light into a beam profiler; and irradiating the transmittedlinear laser light to said semiconductor film.
 7. A method for forming asemiconductor device comprising: forming a semiconductor film comprisingsilicon over a substrate; providing said semiconductor film with anelement for accelerating crystallization of silicon; crystallizing saidsemiconductor film provided with said element by a heat treatment;introducing a CW laser light into an optical system comprising a mirror;shaping said CW laser light into a linear laser light by said opticalsystem; reflecting said linear laser light by said mirror; transmittingsaid linear laser light through said mirror; introducing the transmittedlinear laser light into an energy measuring device; and irradiating thereflected linear laser light to said semiconductor film.
 8. A method forforming a semiconductor device comprising: forming a semiconductor filmcomprising silicon over a substrate; providing said semiconductor filmwith an element for accelerating crystallization of silicon;crystallizing said semiconductor film provided with said element by aheat treatment; introducing a CW laser light into an optical systemcomprising a mirror; shaping said CW laser light into a linear laserlight by said optical system; reflecting said linear laser light by saidmirror; transmitting said linear laser light through said mirror;introducing the reflected linear laser light into an energy measuringdevice; and irradiating the transmitted linear laser light to saidsemiconductor film.
 9. A method according to claim 5 wherein saidelement is selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd,Os, Ir and Pt.
 10. A method according to claim 6 wherein said element isselected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir andPt.
 11. A method according to claim 7 wherein said element is selectedfrom the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt. 12.A method according to claim 8 wherein said element is selected from thegroup consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt.
 13. A methodaccording to claim 1 wherein said semiconductor device is incorporatedinto a liquid crystal display.
 14. A method according to claim 2 whereinsaid semiconductor device is incorporated into a liquid crystal display.15. A method according to claim 3 wherein said semiconductor device isincorporated into a liquid crystal display.
 16. A method according toclaim 4 wherein said semiconductor device is incorporated into a liquidcrystal display.
 17. A method according to claim 5 wherein saidsemiconductor device is incorporated into a liquid crystal display. 18.A method according to claim 6 wherein said semiconductor device isincorporated into a liquid crystal display.
 19. A method according toclaim 7 wherein said semiconductor device is incorporated into a liquidcrystal display.
 20. A method according to claim 8 wherein saidsemiconductor device is incorporated into a liquid crystal display.